The Denver Police Department wants to improve its image with teenagers. A uniformed officer is sent to a school $1$ day a week for $10$ weeks. Each day the officer visits with students, eats lunch with students, attends pep rallies, and so on$.$ There are $18$ schools, but the police department can visit only half of these schools this semester. A survey regarding how teenagers view police is sent to all $18$ schools at the end of the semester. $($Select all that apply.$)$